Electrostatography is a term used to describe various non-impact printing processes which involve the creation of a visible image by the attraction of charged imaging particles to charge sites present on a substrate. Such charge sites, forming what is usually termed the "latent image", can be transiently supported on photoconductors or pure dielectrics, and may be rendered visible in situ or be transferred to another substrate to be developed in that location. Additionally, such charge sites may be the reflection of those structured charges existing within a permanently polarised material, as is the case with ferroelectrics and other such electrets.
Electrostatography encompasses those processes normally known as electrophotography and electrography.
In general, a liquid developer for electrostatography is prepared by dispersing an inorganic or organic colorant such as iron oxide, carbon black, nigrosine, phthalocyanine blue, benzidine yellow, quinacridone pink and the like into a liquid vehicle which may contain dissolved or dispersed therein synthetic or naturally occurring polymers such as acrylics, alkyds, rosins, rosin esters, epoxies, polyvinyl acetate, styrene-butadiene etc. Additionally, to effect or enhance the electrostatic charge on such dispersed particles, additives known as charge directors or charge control agents may be included. Such materials can be metallic soaps, fatty acids, lecithin, organic phosphorus compounds, succinimides, sulphosuccinates etc.
In such developers, whether positively or negatively charged, there is one ingredient of common generic character, namely the carrier liquid. Since the beginning of the history of liquid toners, it has been recognised that certain electrical properties of the carrier liquid are mandatory requirements for the effective functioning of a conventional electrostatographic liquid development process. These are low electrical conductivity and other requirements became obvious, such as the needs for low toxicity, increased fire safety, low solvent power, low odour etc. For these reasons, isoparaffinic-hydrocarbons such as the Isopar range manufactured by Exxon Corporation, the Shellsol range manufactured by Shell Chemical and the Soltrol range manufactured by Phillips Petroleum became the industry standards for liquid toner carriers.
In more recent times, however, certain deficiencies in these isoparaffins have become apparent. Environmental concerns have placed liquid development processes under increasing pressure to reduce or eliminate volatile emissions. Flammability has also become important regarding the more stringent transport regulations existing and anticipated worldwide.
New designs of image fusing stations are placing increased importance on the thermal stability of carrier liquids.
In order to overcome these limitationsother materials applicable to liquid toners have been investigated and of these, silicone fluids are clearly liquids which combine all previously and currently desired properties of a modern liquid toner carrier.
Silicone fluids have been mentioned in the context of liquid toners, e.g. in U.S. Pat. Nos. 3,105,821 to S. W. Johnson, and in 3,053,688 to H. G. Greig. Both of these early patents recognised the virtues of silicone fluids, but the understanding of the functioning of liquid toners at that time was relatively empirical, with those patents teaching simply the mechanical dispersion of a dry toner into the silicone fluid with no regard to chemical compatibility, which in turn governs the final particle size and stability of the dispersion so produced. More recently silicone fluids have again been recognised, as disclosed in JPA-H3-43749.
However, in this application reliance is also placed on mechanical dispersion only and in addition no mention is made of chemical compatibility or most importantly, charge directors, the need for which being well established in the field of liquid electrostatic toners.
It is well known that silicone fluids have low solvent power for plastics and this property is well suited for copy machine components and organic photoconductor life. An unfortunate corollary to this is that many polymers normally used in liquid toners, whether they are chemically prepared such as in U.S. Pat. Nos. 3,990,980 to G. Kosel or more recently 5,112,716 to Kato et al or by conventional dispersion techniques such as in JPA-H3-43749, are either insoluble in or incompatible with silicone fluids. This severely limits the particle size attainable and the stability of dispersions thus prepared due to the inability of such polymers to dissolve in the silicone fluid and subsequently to be adsorbed onto dispersed colorants, providing a steric barrier to their reagglomeration.
Thus the need exists for a stable liquid developer which meets modern environmental demands and yet has the imaging capability required by quality printing standards, namely colour gamut and resolution.
Thus an object of the invention is to provide an electrostatographic toner containing an unadulterated silicone fluid as the carrier liquid.
Another object of the present invention is to provide an electrostatographic toner composition having stable particle charge properties achieved by a silicone fluid compatible charge director.
This invention relates to a liquid developer for electrostatography, comprising particles which may contain or comprise pigments or dyes as colourants and a charge control agent, dispersed in a liquid carrier having an electrical resistance of at least 10.sup.9 -ohm-cm and having a dielectric constant of not more than 3.5. In particular this carrier liquid is further characterised by being silicon containing organic compounds, generally known as silicone fluids.